A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, than the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, so that the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
To reduce the size of features that can be imaged using a lithographic projection apparatus, it is desirable to reduce the wavelength of the illumination radiation. Ultraviolet wavelengths of less than 200 nm are therefore currently contemplated, for example 193 nm, 157 nm, or 126 nm. Also contemplated are extreme ultraviolet (EUV) wavelengths of less than 50 nm, for example, 13.5 nm. Suitable sources of UV radiation include Hg lamps and excimer lasers. EUV sources contemplated include laser-produced plasma sources, discharge sources, and undulators or wigglers provided around the path of an electron beam in a storage ring or synchrotron.
In the case of EUV radiation, the projection system will generally consist of an array of mirrors, and the mask will be reflective. See, for example, the apparatus discussed in WO 99/57596, incorporated herein by reference.
Apparatus which operate at such low wavelengths are significantly more sensitive to the presence of contaminant particles than those operating at higher wavelengths.
Contaminant particles such as hydrocarbon molecules and water vapor may be introduced into the system from external sources, or they may be generated within the lithographic apparatus itself. For example, the contaminant particles may include the debris and by-products that are liberated from the substrate, for example, by an EUV radiation beam, or molecules produced through evaporation of plastics, adhesives, and lubricants used in the apparatus. It will be clear that the term “contaminant particles” encompasses molecular contamination.
These contaminants tend to adsorb to optical components in the system, and cause a loss in transmission of the radiation beam. When using, for example, 157 nm radiation, a loss in transmission of about 1% is observed when only one or a few monolayers of contaminant particles form on each optical surface. Such a loss in transmission is unacceptably high. Further, the uniformity requirement on the projection beam intensity for such systems is generally less than 0.2%. Localized contamination on optical components can cause this requirement not to be met.
EP 1312984, incorporated herein by reference, describes that the cleaning of optical components in a lithographic projection apparatus can be carried out by addition of relatively low partial pressures of stable molecular oxygen to a purge gas which is fed to spaces through which the projection beam travels. As molecular oxygen itself is not effective as cleaning agent, it is used in combination with (E)UV radiation. The (E)UV radiation cracks oxygen to produce oxygen radicals, which are highly effective cleaning agents. With the low concentrations of cleaning agent in the purge gas, the optical components can be cleaned while projecting a mask pattern onto a target portion with acceptable transmission loss due to absorption of (E)UV radiation by oxygen.
After this form of cleaning, the transmission or reflection of the radiation beam is noticeably better than before cleaning. The uniformity has also improved. This form of cleaning is regarded as a highly effective method of cleaning optical components in lithographic projection apparatus. It avoids the use of unstable materials such as ozone. Above all, it prevents very time-consuming dismounting of optical components (e.g. lens elements) out of the lithographic projection apparatus in order to clean the component in a separate cleaning unit.